1. Field of the Invention
The present invention relates to the field of aircraft maintenance, repair and modification.
2. Prior Art
The F-16 aircraft manufactured by General Dynamics is currently in use in substantial numbers by the United States and a number of other Free World countries. It is widely known that, while this aircraft is a high performance aircraft, it has a deep stall characteristic which, together with other factors, results in the flight control system being set to prevent the pilot from commanding the angle of attack to more than approximately 25 degrees. Nonetheless, it is possible to take the F-16 to higher angles of attack with dynamic maneuvering, and several aircraft are believed to have been lost to deep stalls.
The deep stall characteristics of the aircraft are caused by the fact that at an angle of attack of approximately 40 degrees, the pitching moment coefficient goes positive (meaning in the aircraft nose up direction), not going negative (meaning in the aircraft nose down direction) again until an angle attack of approximately 60 degrees is reached. Consequently, once the aircraft reaches an angle of attack in this range, the resulting nose-up pitching moment causes the aircraft to increase angle attack to approximately 60 degrees and to settle there unless appropriate procedures can successfully get the aircraft back down to normal angle of attack operating ranges. The positive pitching moment coefficient, of course, indicates that the center of lift is forward of the center of gravity, a condition which will vary somewhat dependent upon the position of the center of gravity of the aircraft at the time.
The deep stall characteristic can of course be limited if the pitching moment can always be kept negative, even in angle of attack ranges of 45 to 60 degrees. One way of doing this is to redesign the wings of the aircraft, generally enlarging the same to increase the lift aft of the center of gravity. While this is a practical approach for future production, it is not a practical modification for aircraft already in service. It is also generally recognized that had the aircraft been designed with substantially reduced size forebody strakes, the high angle of attack lift of the forward part of the fuselage would be significantly reduced, thereby keeping the center of pressure aft of the center of gravity of the aircraft and the pitching moment coefficient negative. Normally, one would consider such a modification to also be suitable only for new aircraft, and not applicable to the aircraft already in service. The present invention, however, recognizes that because of the construction of the specific aircraft in question and the extent to which the forebody strakes need to be reduced in size, such a modification may readily be made to existing aircraft, as the modification procedure involves the replacement of the appropriate non-primary structural parts of the aircraft.